The invention relates to a method for dissolving noble metals out of segregated ores containing them.
In the state of the art a series of methods are known for dissolving noble metals out of segregated ores. Reference is made in this connection to the blowing of elemental chlorine into a strong hydrochloric acid solution, to dissolving with aqua regia, to electrolytic dissolution of segregated metallic materials and to fusion with sodium chloride/chlorine. A disadvantage of the above-named methods is, for one thing, the use of relatively expensive chlorine gas, the relatively time-consuming introduction of the chlorine gas into the solutions and suspensions, the great contamination of the atmosphere with NOx by the washers and the sometimes problematic work safety.
From what has been said above, the problem is at least partially to remedy the stated disadvantages by means of a novel process. The problem lies especially in the replacement of the chlorine gas with an easy to manage and less expensive source of chlorine, which furthermore can be brought considerably more quickly into the corresponding solutions and suspensions, as well as the substitution of aqua regia and the resultant great reduction of the contamination of the air with NOx by the washers.
This problem is solved according to the invention by a method comprising dissolving noble metals out of segregated ores containing noble metals by a process comprising:
in the presence of noble metal oxides, reduction to the individual metals in finely divided form;
oxidative treatment of the segregated ores and/or the noble metals with HCl and chlorate, the solution or suspension resulting having a concentration of H+ ions of at least 1 mol/l, a concentration of Clxe2x88x92 ions of at least 1 mol/l and a temperature of at least 50xc2x0 C.
In the method of the invention, in dealing with noble metal oxides, a reduction to the individual metals in finely divided form is performed. It is known that noble metal oxides cannot be dissolved directly with hydrochloric acid or oxidants. Therefore the oxides are first reduced with a reducing agent in an alkaline suspension to form the noble metal, which then dissolves in hydrochloric acid plus oxidants.
Fundamentally, the segregated ores and/or the noble metals are treated oxidatively with HCl and chlorate, and the developing solution or suspension has a concentration of H+ ions of at least 1 mol/l, a Clxe2x88x92 ion concentration of at least 1 mol/l, and a temperature of at least 50xc2x0 C.
One known method is to dissolve noble metals in hydrochloric acid/chlorine. In a hydrochloric acid solution, chlorine can be formed by the reaction of chlorate with hydrochloric acid which then oxidizes the noble metals. A H+ ion concentration of 1 mol/1 and a minimum temperature of 50xc2x0 C. are necessary, however, in order for the chlorate to react chemically.
Potassium chlorate or sodium chlorate can be used, for example, as chlorate sources.
It is an advantage if the temperature of the solution or suspension formed is no more than +95xc2x0 C., since above +95xc2x0 C. explosive ClO2 (chlorine dioxide) can form.
It is furthermore advantageous if the solution or suspension has at least 3 mol/l of H+ ions, since in this way the noble metals are dissolved rapidly and virtually completely.
It is advantageous, furthermore, if the oxidation is performed under potential control, since in this manner the end of the oxidation is relatively easy to know and thus a targeted control of the process is possible.
It is furthermore advantageous if, in the case of noble metal oxides, the reduction is performed by means of formic acid or hydrazine, since these reducing agents have proven effective in practice. The noble metal oxides are suspended in alkaline solution, especially caustic soda solution and treated with the reducing agent at elevated temperature. Formic acid is used when noble metal oxides are present together with noble metals, and hydrazine in the case of merely oxide materials.
If the reduction is performed by means of hydrazine, it is advantageous if the noble metals are washed free of hydrazine so as not to interfere stubbornly with the following oxidation.
The following specifications are especially advantageous since they have proven effective in practice:
The segregated material containing noble metals and/or the noble metals are suspended in 3 to 5 molar hydrochloric acid before the oxidative treatment with HCl and chlorate. The suspension or solution of the segregated material containing noble metals is heated to +70xc2x0 C. to +80xc2x0 C. The chlorate is added as an aqueous 4.5 to 5.0 molar chlorate solution.